A Nuclear Explosive-Formed Penetrator is a Casaba Howitzer with a thick inverted cone of metal in front of it. The nuclear force, through the Monroe effect, is used to accelerate a metal plate of several tons to several kilometers per second.
It will be much faster than hypersonic missiles, impervious to point defenses and probably have to range to destroy targets over the horizon.
Aerodynamic heating is not a big problem when the metal is already half-molten. Drag can be compensated for through sheer momentum - the mass required to prevent the nuclear explosion from simply vaporizing the metal plate has a side effect of having the NEFP projectile mass several tons.
This is a better idea than a CH in atmosphere, but I'm skeptical of whether this sort of thing is really a viable naval weapon in the first place. OTH positional fixes are in practice not precise enough to direct a weapon like this- there are good reasons actual militaries prefer supersonic AShMs with terminal guidance to things like railguns, even if they are slightly more susceptible to CIWS fire. You also have no possibility of getting plunging hits and low/no possibility of below-waterline hits. If you're going nuclear in an earthbound confrontation you may as well just use a ballistic missile (also impervious to point defence) to deliver your nuclear material and frag the entire battlegroup instead of hoping to poke above-waterline holes in a few of them with EFPs.
A fusion NEFP can reach theoretically 30000km/s in space, or more, but I guess the projectile won't survive through the atmosphere. If you increase the projectile mass and tone down the velocity, say, to something like Mach 12, you can cross the horizon in about 5 seconds.
By increasing the projectile mass, I'm talking about blobs of metal massing several hundred kilos to several tons, that reach the target still travelling at hypersonic speeds, and don't so much pierce as punch a tunnel through the target.
Also, there's a slim politico-juridistical chance that 'launching' projectiles using nuclear energy instead of detonating a nuclear warhead next to the target bypasses the nuclear proliferation laws.
Also, there are some wacky things you can do with a CH-NEFP. Fly your missile into the upper atmosphere, then have it fire from an altitude of 20km. Launch it to impact in the water next to a ship, and have the sudden momentum transfer of a multi-ton hypersonic projectile into the surrounding water just crush the ship's hull, and so on.